Studies have been conducted on the control of shedding of outer segment tips and their phagocytosis by the pigment epithelium. Animals were killed, the eyes processed for electron microscopy, and phagosomes were counted by light microscopy. The dynamics of the circadian rhythm of shedding were studied by maintaining albino rats in complete darkness, constant light (CL), or under different environmental light cycles. CL inhibited shedding but did not erase the underlying rhythm controllng shedding. The minimum dark period required for shedding was determined. A series of experiments was conducted in which bilateral optic nerve section was employed to determine if the CNS participated in the control of the shedding. The shedding rhythm continued after the optic nerves were severed, although it could no longer adapt to altered lighting cycles. The surgery had no effect on the response of the eyes to CL exposure. Finally, a burst of shedding could be triggered at any time of the day by exposing the animal to CL followed by two hours darkness and one and one-half hours light, although the magnitude of the induced shedding depended upon the time of the day.